The invention relates to molding elements intended to equip molds for molding articles or objects made of rubber and in particular treads for tires. It relates more specifically to molding elements for molding patterns of non-demoldable shape.
A non-demoldable pattern is intended to mean a pattern which has at least one undercut in its shape which means that if the molding element used for molding said pattern is trapped in rigid material, then there is no possible sliding between the element and the surrounding material because the part with the undercut opposes such sliding. The term rigid is intended to mean a material which does not allow enough elastic deformation for it to be possible for the element to be demolded, unlike the elastomeric material used, for example, to make a tire tread. In the latter case, and although the material based on a rubber compound is flexible enough to deform elastically and thus allow the demolding of the non-demoldable molding element, there may, however, be substantial deformation which, during the molding, may cause breaks in the molded material. These deformations of the molding material are all the greater if each molded pattern has at least one part with an undercut.
It has been observed that to meet an increase in performance with tires over time there is an increasing demand for tire treads which have patterns in which at least part of the tread pattern has an undercut.
It is known, for example from U.S. Pat. No. 4,794,965, that to improve the wear life of a tread with relief elements, these can be provided with a plurality of incisions which, on the running area of the tread, present straight lines, and in cross-section present broken or undulating lines over the depth.
Furthermore, and to avoid the appearance of the fatigue cracks at the bottom of the incision, it is known, see for example the patent U.S. Pat. No. 2,121,955, for the bottom of the incisions of a tire tread to be provided with a widened part forming a channel of an appropriate cross-section (which channel can also play a part in removing the water present on the area on which the tire is running). To make such an incision use is generally made of a molding element in the form of a somewhat thin metal blade which at one of its ends has a bulge intended to mold the widening of the pattern at the bottom of the incision.
With the purpose of overcoming the drop in grip during the life of a tire, U.S. Pat. No. 4,994,126, describes a tread which has incisions which, in cross-section, present lines in the shape of a fork with at least two branches, so that when said tire has been partially worn, a greater number of incisions appear. To produce such incisions, the use of rigid and non-deformable blades, generally metal, is known, the cross-sectional geometry of which is equivalent to the desired geometry of the incision. It will be readily understood that for some geometries of such blades, it is particularly difficult to extract them from the tread after molding, the branches of the block blade having to force their way out by parting the walls of that part of the incision which opens to the area of the tread.
Furthermore, when a great many non-demoldable molding elements are provided at the area of a mold, and such that they are fairly close together, it has been seen that the demolding force becomes very high; this results in an increased expenditure of energy and in an increase in the risk of material being torn out of the molded strip as a result of the demolding operation.